In an analog telephone line environment, the telephone company central office supplies a ring voltage of approximately 90 volts alternating current (AC) to activate an alerting mechanism on the analog telephone to an incoming call. The ring voltage is applied to the two incoming telephone lines, known as the "tip" and the "ring", and causes the analog telephone to ring (or provide some other type of indication of the incoming call) until the handset is removed and the telephone goes off hook.
This 90 volt AC ring voltage is not supplied by the telephone company in an Integrated Services Digital Network (ISDN) line environment. When a data packet notifies an ISDN data communications device (DCD) of an incoming call, the DCD should generate a ring voltage to activate the alerting mechanism. Existing power supplies used to generate such ring voltages include sources such as high voltage direct current (DC) converters, power supplies, and batteries for supplying a high voltage direct current. The high voltage direct current may then be coupled to the tip and ring through a switch that alternately exchanges the polarity to the tip and ring at the specified ring frequency, thereby applying a high voltage alternating current to the alerting or ringing mechanism. In other existing devices, the high voltage direct current is coupled to the tip and ring alternately with a ground voltage, to cause a reversal of current flow through the ringing mechanism at the specified ring frequency.
Existing methods and apparatus for generating ring voltages tend to have a low efficiency, which may both waste energy and require additional heat sinks and other means for heat dissipation on the printed circuit board. In addition, the existing methods and apparatus may require additional parts, with corresponding additional and significant board space, having a further consequence of adversely impacting both the cost and ease of manufacture of the ISDN data communications device. A need has continued to exist to provide a more energy efficient and compact apparatus and method to provide a ring voltage in data communications devices in an ISDN line environment, which also may be manufactured efficiently and economically.